The invention refers to a configuration for polishing diskshaped objects including a polish platen and a polish head with a backing film, especially for polishing semiconductor wafers.
In semiconductor wafer manufacturing, the wafer obtains an uneven surface in the process of depositing and growing of different films and materials. Especially with small feature sizes, these non-planarities of the wafer surface lead to alignment errors during lithography. A common process to improve uniformity is a polishing of the wafer surface. The backside of the wafer is held by a polish head of the polishing tool and the front side to be polished is moved across a pad on the platen of the polish tool. The head is rotated around its own axis as well as around another axis together with multiple polishing heads. A slurry including a specific composition of different ingredients interacts between the wafer and the polish pad to provide appropriate polish effects as well as cooling. This process of polishing semiconductor wafers is referred to as Chemical Mechanical Polishing (CMP). CMP may also be applied to other manufacturing processes of disk-shaped objects such as CDs or flat panel substrates or the like.
One critical factor in CMP is uniformity. When the backside of the wafer within the polishing head is supported by a polyurethane film there are inherent disadvantages. Polyurethane is inhomogeneous, having zones of different hardness and different density. This is due to the fact that a polyurethane backing film is fabricated by foaming, which is poorly controllable. The film is cut off from bulk material. The polyurethane backing film has varying compressibility so that the force applied to the wafer backside results in an inhomogeneous polishing result on the polished surface of the wafer.
Another disadvantage of polyurethane backing films is that the film sucks liquid into its holes of the foam structure. Especially those holes that are cut and open on the surface of the backing film accept liquid from the slurry. The sucking of liquid into the polyurethane backing film results in a further introduction of non-uniformity to the polishing process.
In U.S. Pat. No. 6,012,964 to Arai et al., a CMP apparatus is disclosed that seeks to improve uniformity. The polish head has a soft backing pad facing the backside of the wafer to be polished that is supported by a hard sheet. The hard sheet is applied with air pressure to adjust the shape of the hard sheet and compensate uniformity problems. The backing film can be made of various materials including silicone rubber. The shape of the surface of the hard sheet is controlled by the air pressure applied to the pressure chamber behind the hard sheet. Because the surface shape of the hard sheet is not rigid and depends on the pressure applied to the pressure chamber, uniformity problems remain.
It is accordingly an object of the invention to provide a configuration for polishing disk-shaped objects that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that provides better uniformity during polishing.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a configuration for polishing disk-shaped objects having a first surface and a second surface opposite the first surface, including a platen adapted to contact the first surface of an object to be polished and a polish head having a backing film removably attached to the polish head and adapted to directly contact the second surface, the backing film being of silicone and a rigid support element carrying the backing film.
In accordance with another feature of the invention, the object is a semiconductor wafer, in particular, a silicon wafer.
A configuration for polishing disk-shaped objects includes a platen that contacts a first surface of the object that is to be polished, a polish head that includes a backing film that is removably attached to the polish head to directly contact a second surface of the object opposite to the surface to be polished, and a support element carrying the backing film that is rigid, the backing film being of silicone.
The polish head according to the configuration of the invention has a rigid, incompressible support for the backing film, the surface of which having a constant shape. For better uniformity and controllability of the polish process, the backing film that holds the backside of the wafer is formed of silicone. Compared to a polyurethane backing film that is made from a foam, silicone has the advantage that it is made from a liquid. The liquid is introduced into a molding form. Compared to the poorly controllable process of foaming, the result of molding a backing film can be tightly controlled.
Because the silicone backing film is made from a liquid phase, the finished backing pad is an inherently uniform material having the same density and compressibility throughout its body. A silicone backing pad does not leak any air so that it can be attached to the polish head by underpressure. There is no need to glue the backing film to its support on the polish head so that uniformity is further improved. By avoiding any glue, a backing film can easily be replaced by a new silicone backing film at the end of its lifetime. Preferably, the support for the backing film is a ceramic plate or chuck that is rigid and provides an incompressible support for the backing film, the surface of which always having the same constant shape. A ceramic chuck is amorphous so that an evenly distributed vacuum can be provided through the ceramic chuck to hold the backing film onto the chuck. This vacuum is provided by a vacuum chamber at the surface of the ceramic chuck that is opposite to the backing film. The ceramic chuck contacts the backing film directly.
In accordance with a further feature of the invention, the vacuum generator supplies a vacuum to the support element to hold the backing film on the support element.
In accordance with an added feature of the invention, the polish head has a first vacuum chamber supplying a vacuum to the ceramic support element, a second vacuum chamber above the first vacuum chamber, and a plurality of tubes projecting from the second vacuum chamber through the first chamber, through the ceramic support element, and through the backing film to end above the second surface of the object, the tubes supplying a vacuum to the second surface of the object to hold the object onto the backing film.
There is another vacuum chamber that provides a vacuum to the backside of the wafer through tubes. The tubes are projecting from this latter vacuum chamber through the chamber providing the vacuum to the ceramic chuck, are protruding through the ceramic chuck, are protruding through the silicone backing film, and, finally, are ending close to the back surface of the wafer.
In accordance with an additional feature of the invention, the backing film has a surface adapted to directly contact the second surface of the object, the surface of the backing film having a microstructure with a plurality of enhanced portions contacting the second surface of the object and a plurality of recessed portions not contacting the second surface of the object.
Because the silicone backing film does not leak any air, the wafer is tightly attached to the backing film. To facilitate the removal of the wafer, the surface of the backing film facing the backside of the wafer is provided with a microstructure. The microstructure has enhanced elements that contact the wafer backside and has recessed elements that do not contact the wafer backside. Thereby, the contact area is reduced. The adhesive force holding the wafer at the backing film is, thereby, also reduced. With an appropriate relation between enhanced elements and recessed elements of the microstructure it can be accomplished that the wafer does not stick to the backing film any more when the vacuum for holding the wafer is switched off. The microstructure is applied to the surface of the backing film by a complementary shape in the molding form. This shape can be produced by applying a lithography step comparable to a lithography step for patterning a semiconductor wafer surface during semiconductor wafer manufacturing. To achieve the microstructure through lithography the surface of the molding is subjected to a photoresist that is exposed to optical radiation. The structure is formed by etching the unprotected areas of the molding surface.
In accordance with yet another feature of the invention, the surface of the backing film is concave, convex, or U-shaped defining a macroscopic recess in a center thereof, the recess not contacting the object.
By molding the silicone backing film, it is possible to provide any macroscopic shape to the backing film so that any already known non-uniformity of the wafer to be polished is compensated for. For example, the wafer needs to be polished more in the center than at its end or vice-versa. To compensate for this non-uniformity of the wafer, the backing film can be convex, e.g., the backing film is thicker at its center than at its circumference and can be formed concave, e.g., thinner at its center than at its circumference, respectively. Alternatively, the backing film has a macroscopic recess in its center and projecting parts at its circumference. The center recess does not touch the backside of the wafer whereas the projecting parts contact the wafer. Thereby, a vacuum is enclosed at the center of the backing film. The projecting parts can also be provided with the microstructure. Any other conceivable shape of the surface facing the backside of the wafer can be manufactured by an appropriate complementary shape of the molding.
Another solution to compensate for already known non-uniformities of the wafer to be polished is to provide the backing film with zones of different hardness. Preferably, those zones have concentric shape with respect to the center of the backing film, although different radial sections of the backing film may also have different hardness. All depends on the flexibility of the molding process to fabricate the silicone backing film. The hardness is controllable by the addition of solid particles, like silicon particles or aluminum oxide particles. Other solid materials are possible. Depending on the electrical behavior of the added material, any electrostatic fields introduced during the polish process can be eliminated.
With the objects of the invention in view, there is also provided a configuration for polishing semiconductor wafers having a first surface and a second surface opposite the first surface, including a platen adapted to contact the first surface of a wafer to be polished and a polish head having a backing film removably attached to the polish head and adapted to directly contact the second surface, the backing film being of silicone, a rigid support element carrying the backing film, and a vacuum generator holding the backing film on the support element.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for polishing disk-shaped objects, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.